A severe thunderstorm packing winds estimated at more than 80 mph spawned a tornado, injured at least six people and caused significant damage to homes in western Michigan, officials said Monday, and storms with tornadoes damaged some homes in Iowa.

A fast-moving thunderstorm intensified in Michigan late Sunday over Kentwood, just south of Grand Rapids, the National Weather Service said.

Touching down in Byron Center, the twister traveled just over 6 miles and was 300 to 400 yards wide, according to the weather service website.

The tornado's winds reached the 100-110 mile per hour range, weather service forecaster Brian Meade told The Associated Press on Monday. He said the impact was serious but limited in area.

"It's not extensive damage, but there was some heavy damage within it," Meade said.

The tornado knocked down trees and power lines, ripped the roofs of a number of houses and sent two people to hospitals for treatment after their homes collapsed, Kentwood Mayor Stephen Kepley said in a statement. Many roads remained impassible Monday afternoon because of fallen trees.

The American Red Cross opened a shelter about 1 a.m. Monday at a Kentwood recreation center, and about 60 people stayed overnight, said Kent County emergency management coordinator Jack Stewart. He said about 20 people were still there Monday afternoon.

In all, six people were hurt, with none of the injuries appearing life-threatening, Stewart said.

In Iowa, several tornadoes were reported and at least two homes were damaged after storms passed through central Iowa. The same possible tornado tracked through both Grundy and Tama counties, the weather service said, with a separate storm hitting Jasper County.

In North Dakota, weekend storms caused heavy damage at a Missouri River recreation area south of Bismarck and injured people at a lake southwest of the city. Several tents and vehicles were destroyed by strong winds and falling trees at Kimball Bottoms on Saturday night.

In Michigan, Jan Collins told The Grand Rapids Press that she heard the wind blow and then, an enormous boom as a tree toppled onto her family's garage.

"There was this rumbling sound, and it was raining especially hard, before the wind came out of nowhere," Collins said. "I think the car will be OK -- the tree kind of just rested on top."

Flood warnings were in effect until Tuesday morning in Michigan's Gratiot, Ionia and Montcalm counties, where some roads were washed out, the weather service said.

CMS Energy Corp.'s Consumers Energy unit said Monday morning that roughly 20,000 of its customers lost power. About 9,600 of those homes and businesses are in Kent County, including Kentwood.